An organic electroluminescent device has a multilayer structure comprising (i) a luminescent layer comprising a light emitting compound, (ii) a hole transport layer and an electron transport layer, which sandwich the luminescence layer, and (iii) an anode and a cathode, which sandwich the hole transport layer, the luminescent layer and the electron transport layer. The organic electroluminescent device utilizes light emission (fluorescence or phosphorescence) occurring at deactivation of an exciton formed by the recombination of electron with hole, which are injected in the luminescent layer.
In recent years, a wide spread attention is attracted to an organic electroluminescent device for next-generation flat panel displays. This is because, first, an electroluminescent device can be made into a thin film and be rendered light in weight; secondly, power consumption is low due to the spontaneous light emission; and thirdly, the device structure is simple and thus the production cost is low. Various methods can be adopted for the production thereof, which include, for example, vacuum deposition, spin coating, ink-jet printing, off-set printing and thermal transfer printing.
Now various mobile devices such as cell phones, mobile music devices, and personal digital assistant (PDA) are widely used. However, if mobile devices can be larger in size or more precise, organic electroluminescent devices are expected to be used in, for example, flat panel displays, lighting systems with a surface-light-emitting source, flexible paper-like displays, wearable displays and transparent see-through displays. Its use is expected to be rapidly spread.
However, an organic electroluminescent device still has many technical problems to be solved. Especially its driving voltage is high and its efficiency is low, and therefore, its power consumption is high. In addition, the high driving voltage often causes shortening of lifetime of the organic electroluminescent device.
The above-mentioned technical problems arise due to the property of the material constituting the organic electroluminescent device, especially the property of electron transport material. Many materials including triarylamine derivatives have been proposed as a hole transport material, but, only several reports are found as to the electron transport material. Tris(8-quinolinolato)-aluminum (III)(Alq) is already put in practical use as an electron transport material, but, its property is poor as compared with a hole transport material such as, for example, N,N′-bis(1-naphthyl)-N,N′-diphenyl-4,4′-biphenyl (NPD), and an organic electroluminescent material comprising the electron transport material has also poor property.
As other electron transport materials, there can be mentioned oxadiazole derivatives (patent document 1), quinoxaline derivatives (patent document 2), triazole derivatives (patent document 3), silacyclopentadiene derivatives (patent document 4), quinoline derivatives (patent document 5), benzimidazole derivatives (patent document 6) and benzothiazole derivatives (non-patent document 1). However, organic electroluminescent devices comprising these electron transport materials still have problems in practice in that their driving voltage is high, the film is readily crystallized, and their lifetime is short.
Recently, 1,3,5-triazine derivatives have been proposed as materials for organic electroluminescent devices (patent documents 7 and 8). These proposed triazine derivatives have benzoazol moieties in 2, 4 and 6 positions of the triazine ring, and thus, their chemical structures are clearly distinguished from the 1,3,5-triazine derivative of the present invention.
Use of 1,3,5-triazine derivatives for organic electroluminescent device is described in patent documents 9 to 12. These 1,3,5-triazine derivatives have two phenyl substituents in o- and p-positions or m- and p-positions. In the patent documents, 1,3,5-triazine derivatives having two phenyl substituents in o- and p-positions or m- and p-positions are described. However, 1,3,5-triazine derivatives having two phenyl substituents in 3- and 5-positions such as the 1,3,5-triazine derivatives of the present invention are not described at all.
Use of 1,3,5-triazine derivatives for organic electroluminescent device is described further in patent document 13. The 1,3,5-triazine derivatives described therein have aromatic heterocyclic substituents in all of the 2-, 4- and 6-positions of the 1,3,5-triazine derivatives, and therefore, the 1,3,5-triazine derivatives are different from the 1,3,5-triazine derivatives of the present invention.
One example of 1,3,5-triazine derivative used for organic electroluminescent device is mentioned in patent document 14 (page 27, compound No. C-8). The exemplified 1,3,5-triazine derivative has the same aromatic hydrocarbon substituents in the 2-, 4- and 6-positions of the 1,3,5-triazine derivative, therefore, it is distinguished from the 1,3,5-triazine derivatives of the present invention.
Another example of 1,3,5-triazine derivative used for organic electroluminescent device is mentioned in patent document 15 (page 8, compound No. 1). The exemplified 1,3,5-triazine derivative has the same 4-monosubstituted phenyl groups in the 2-, 4- and 6-positions of the 1,3,5-triazine derivative, therefore, it is distinguished from the 1,3,5-triazine derivatives of the present invention.
Use of 1,3,5-triazine derivatives for organic electroluminescent device is described further in patent document 16. These 1,3,5-triazine derivatives have mono-substituted phenyl groups in the 2-, 4- and 6-positions of the 1,3,5-triazine derivatives, therefore, it is distinguished from the 1,3,5-triazine derivatives of the present invention.
Use of 1,3,5-triazine derivatives for organic electroluminescent device is described further in patent document 17. The 1,3,5-triazine derivatives described therein have substituted phenyl groups in 2-, 4- and 6-positions thereof, but, the positions of substituents in each phenyl group, at which the substituents are bonded to the phenyl group, are not defined. Patent document 17 is silent, in the description thereof including working examples, on the 1,3,5-triazine derivative of the present invention, which has a 3,5-disubstituted phenyl group as one substituent at 2-position of the triazine ring, and further has two aromatic hydrocarbon groups as substituents at 4- and 6-positions of the triazine ring. Further, the 1,3,5-triazine derivative of the present invention cannot be produced by the processes described in the working examples in patent document 17.    Patent document 1: JP H6-136359 A    Patent document 2: JP H6-207169 A    Patent document 3: WO95/25097    Patent document 4: JP 2005-104986 A    Patent document 5: JP 2006-199677 A    Patent document 6: WO2004/080975    Patent document 7: JP H7-157473 A    Patent document 8: JP 2003-303689 A    Patent document 9: U.S. Pat. No. 6,057,048    Patent document 10: U.S. Pat. No. 6,229,012    Patent document 11: U.S. Pat. No. 6,225,467    Patent document 12: JP 2004-63465 A    Patent document 13: JP 2003-45662 A    Patent document 14: JP 2001-143869 A    Patent document 15: JP 2003-282270 A    Patent document 16: JP 4106974 B    Patent document 17: JP 2007-137829 A    Non-patent document 1: Applied Physics Letters, vol. 89, 063504, 2006.